Vital signs sensor and method of measuring vital signs of a user

ABSTRACT

PPG sensor emits light at at least three wavelengths (Y 1 -Y 3 ) and detects the reflected light. The PPG sensor comprises a motion correction unit for correcting motion artefacts from the detected light signals by subtracting the output signal of the detected light at the second wavelength (Y 2 ) from an average of an output signal of the detected light at the first and third wavelength (Y 1 , Y 3 ). The three wavelengths (Y 1 -Y 3 ) are arranged around 550 nm. The second wavelength (Y 2 ) is arranged equal distantly between the first and third wavelength.

TECHNICAL FIELD

Various embodiments relate to a vital signs sensor as well as to amethod of measuring vital signs of a user.

BACKGROUND

Optical heart rate sensors are well known to monitor or detect vitalsigns like a heart rate of a user. Such a heart rate sensor can be basedon a photoplethysmographic (PPG) sensor and can be used to acquire avolumetric organ measurement. By means of optical pulse sensors or pulseoximeters, changes in light absorption of a human skin are detected andbased on these measurements a heart rate or other vital signs of a usercan be determined. The PPG sensors comprise a light source like a lightemitting diode (LED) which is emitting light into the skin of a user.The emitted light is scattered in the skin and is at least partiallyabsorbed by the blood. Part of the light exits the skin and can becaptured by a photo detector. The amount of light that is captured bythe photo detector can be an indication of the blood volume inside theskin of a user. A PPG sensor can thus monitor the perfusion of blood inthe dermis and subcutaneous tissue of the skin through an absorptionmeasurement at a specific wave length. If the blood volume is changeddue to the pulsating heart, the scattered light coming back from theskin of the user is also changing. Therefore, by monitoring the detectedlight signal by means of the photo detector, a pulse of a user in hisskin and thus the heart rate can be determined. Furthermore, compoundsof the blood like oxygenated or de-oxygenated hemoglobin as well asoxygen saturation can be determined, when at least two colors are used.

The pulse signal of a heart beat can be detected by photoplethysmographyPPG which is measuring a variation in the blood volume of the humantissue. In a PPG sensor, light emitting diodes e.g. at wavelengthsbetween 520 nm (green) and 850 nm (infrared) are used to emit light ontothe skin of a user. Transmission type PPG measurements are performedwith light at wavelength ranges of 650-850 nm while reflective type PPGsensing is used at 520-570 nm.

Light is scattered in the skin of the user and some of the light isabsorbed by blood. The reflected light exits the skin and can bedetected by a photo diode. The output signal of the photo diode cantherefore be an indication of the blood volume as well as the variationof the blood volume, i.e. the pulse in the skin of a user.

FIG. 1 shows a graph indicating an output signal of a PPG sensoraccording to the prior art without a movement. In the graph, the heartrate or pulse signal is clearly detectable.

However, in the presence of movement, the output signal of the PPGsensor can be distorted.

FIG. 2 shows an output signal of a PPG sensor according to the prior artwithout a motion and in the presence of motion. In FIG. 2, the outputvoltage V of the PPG sensor is depicted over time. In the region A1 aswell as in the region A3, no motion is present. Motion is, however,present in the region A2. As can be seen in the region A2 because of theinfluence of motion, the pulse signals are harder to be determined. Alarge part of the artefacts in the region A2 is due to blood in theveins of a user as the blood pressure is smaller in the veins.

U.S. Pat. No. 7,727,159 B2 discloses a PPG sensor with a motion artefactcorrection capability.

SUMMARY

Various embodiments described herein provide a vital signs sensor withan increased signal to noise ratio by eliminating motion artefacts inthe output signal of the vital signs sensor.

According to various embodiments, an optical vital signs sensor isprovided to measure or determine vital signs of a user. The opticalvital signs sensor can be a photoplethysmographic sensor (PPG). A lightsource is configured to generate at least three wavelengths which aredirected towards a skin of the user. The sensor also comprises a photodetector unit configured to detect an intensity of light at the at leastthree wavelengths, wherein said light is indicative of a reflection oflight emitted in or from the skin of the user. The sensor also comprisesa motion correction unit configured to correct motion artefacts from thelight intensity detected by the photo detector by subtracting the lightintensity detected at the second wavelength from an average of the lightintensity detected at the first wavelength and that at the thirdwavelength. The first, second and third wavelengths are arrangedapproximately around 550 nm. The second wavelength is arranged at anequidistant position or wavelengths between the first and secondwavelength. As an example, the first wavelength is 530 nm, the secondwavelength is 550 nm and the third wavelength is 570 nm.

According to various embodiments, the second wavelength corresponds to asum of the first and third wavelength divided by 2. If the first, secondand third wavelength is selected accordingly, this results in an easyand effective motion artefact correction.

According to various embodiments, the second wavelength corresponds toapproximately 550 nm. Accordingly, the first wavelength may be 530 nmwhile the third wavelength is 570 nm. Alternatively, the firstwavelength may be 540 nm while the third wavelength can be 560 nm.

According to various embodiments, a method of measuring or determiningvital signs of a user with an optical vital signs sensor configured tomeasure or determine vital signs of a user is provided. The opticalvital signs sensor is a PPG sensor. Light is generated at at least threewavelengths and is directed to what a skin of a user. An intensity oflight which is indicative of a reflection of light emitted in or fromthe skin of a user is detected at the at least three wavelengths. Motionartefacts are corrected from the detected light by subtracting the lightintensity detected at the second wavelength from an average of the lightintensity detected at the first wavelength, and at the third wavelength,The first, second and third wavelengths are arranged approximatelyaround 550 nm. The second wavelength is arranged equidistantly betweenthe first and third wavelength.

According to various embodiments, a computer program for monitoring aheart rate of a user in an optical vital signs sensor as defined aboveis provided. The computer program comprises program code means forcausing the optical vital signs sensor to carry out the steps of themethod measuring or determining vital signs of a user when the computerprogram is run on a computer controlling the optical vital signs sensoror when the computer program is run in the optical vital signs sensor.

According to various embodiments, the vital signs sensor comprises a LEDbased PPG sensor. The LED light penetrates the skin of the user, isreflected and some of it can reach a photo detector. The output of thephoto detector can be used to monitor a blood volume fraction and bloodcompounds like oxygenated and de-oxygenated hemoglobin. In particular,the amount of absorption or reflectance of the light from the LED lightsource can be used to determine the heart rate as well as the bloodvolume fraction or blood compounds. The heart rate relates to the bloodvolume fraction. Furthermore, the PPG sensor according to variousembodiments is therefore an optical sensor allowing a non-invasivemeasurement of vital signs of a user.

According to various embodiments a PPG sensor is provided for measuringor detecting a heart rate of a user. The PPG sensor comprises at leastone light source such as a LED and at least one photo detector such asphoto diode. The signal received by the photo diode is processed todetermine the heart rate of a user. In order to correct any motionartifacts which were generated by a motion of the user while wearing thePPG sensor, a light at three different wavelengths which are equidistantfrom each other and which are arranged around 550 nm are emitted by thePPG sensor. In order to remove the motion artifacts from the outputsignals of the photo detector the output signal of the photo detectorand the second wavelength is subtracted from the average output signalof the photo detector at the first and at the third wavelength.

It shall be understood that some embodiments can also be a combinationof the dependent claims or above embodiments or aspects with respectiveindependent claims.

These and other aspects be apparent from and elucidated with referenceto the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following drawings:

FIG. 1 shows a graph of an output signal of a PPG sensor according tothe prior art,

FIG. 2 shows an output signal of a PPG sensor according to the priorart,

FIG. 3 shows a basic representation of an operational principle of avital signs sensor according to various embodiments,

FIG. 4 shows a graph indication the intensity of light reflected from askin of a user according to various embodiments,

FIG. 5 shows a graph indicating the dependence of the amplitude of anoutput signal versus the wavelength of the light from a PPG sensor,

FIG. 6 shows a graph indicating a spectrum of light reflected by bloodas well as a spectrum of light reflected by the tendon of the user,

FIG. 7 shows a block diagram of an optical vital signs sensor accordingto various embodiments, and

FIG. 8 shows a graph of the output signal of a PPG sensor without andwith a motion correction according to various embodiments.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 3 shows a basic representation of an operational principle of avital signs sensor. In FIG. 3, a heart rate sensor 100 with its contactsurface 101 is arranged or placed on for example an arm of a user. Thevital signs sensor can be based on a photoplethysmograph PPG sensor. Thecontact surface 101 can be directly placed onto the skin 1000 of theuser. The heart rate sensor 100 comprises at least one light source 110and at least one photo detector 120. The light source 110 emits lighte.g. via the contact surface 101 onto or in the skin 1000 of a user.Some of the light is reflected and the reflected light can be detectedby the photo detector 120. Some light can be transmitted through tissueof the user and be detected by the photo detector 120. Based on thereflected light, vital signs of a user like a heart rate can bedetermined. A motion correction unit 130 is configured to correct motionartefacts from the output of the at least one photo detector 120 toimprove the signal-to-noise ration and to determine a heart rate orother vital signs of the user.

The output signal of the PPG sensor gives an indication on the bloodmovement in vessels of a user. The quality of the output signal of thePPG sensor can depend on the blood flow rate, skin morphology and skintemperature. In addition, optical losses in the PPG sensor may also havean influence on the quality of the output signal of the PPG sensor. Theoptical efficiency of the PPG sensor can depend on reflection losseswhen light penetrates from one media into another. Furthermore,scattering of light at the surface of the skin of the user may also havean influence on the optical efficiency of the PPG sensor.

The PPG sensor or optical vital signs sensor according to variousembodiments can be implemented as a wrist device (like a watch or smartwatch). The optical vital signs sensor can also be implemented as adevice worn behind the ear of a user, e.g. like a hearing aid.

Optionally the PPG sensor according to various embodiments can also beimplemented as non-invasive sensor, a non-contact or contact-lesssensor. Such a contact-less sensor can comprise at least two(non-contact) optical fibers (one optical fiber as transmitter or lightsource and one optical fiber as receiver) and can be used to detect thevital signs of a user.

FIG. 4 shows a graph indicating the intensity of light reflected from askin of a user according to various embodiments. In FIG. 4, a spectrumof light I as reflected from a skin 1000 of a user is depicted over thewavelength W (nm). However, the amplitude of the reflected light canchange due to the pulsation of blood as well as the movement of the useror a relative movement of the PPG sensor and the user.

FIG. 5 shows a graph indicating the dependence of the amplitude PA of anoutput signal versus the wavelength W (nm) of the light from a PPGsensor. In FIG. 5, an amplitude of reflected light due to a motion PPG2as well as an amplitude of reflected light due to the pulse PPG1 isdepicted over the wavelength.

As can be seen from FIG. 5, the amplitude of the pulses are inparticular high and distinct at wavelengths around 550 nm.

FIG. 6 shows a graph indicating a spectrum of light reflected by bloodas well as a spectrum of light reflected by the tendon of the user. InFIG. 6, the upper curve shows the dependence on the spectrum of light Ias reflected by the blood of a user. The lower curve shows a spectrum oflight I as reflected by the tendon of a user. While the light asreflected by the blood has two distinct peaks, the light as reflected bythe tendon has an approximate linear decline with increasing wavelengthW (nm).

FIG. 7 shows a block diagram of an optical vital signs sensor accordingto various embodiments. The optical vital signs sensor 100 may comprisea contact surface 101 which can be placed in direct contact with theskin 1000 of a user. The optical vital signs sensor comprises a lightunit 110 which can have three light emitting diodes 111-113. These threelight emitting diodes 111-113 may emit light 111 a, 112 a, 113 a atthree different wavelengths. Alternatively, the light unit 110 may alsocomprise one tunable light emitting diode which can emit light 111 a,112 a, 113 a at three different wavelengths.

The optical vital signs sensor 100 furthermore comprises a photodetector unit 120 which is able to detect the reflected light 121 a-123c. The light unit 110 can be able to emit light 111 a-113 a at threewavelengths. The photo detector 120 may comprise three different photodiodes 121-123 which are able to detect the reflected light at the threedifferent wavelengths 121 a-123 b. The output of the photo detector 120is forwarded to the motion correction unit 130 which is performing amotion correction on the output signals. The motion correction 130serves to remove motion artefacts from the output signal of the photodetector.

The three different wavelengths may be Y₁, Y₂ and Y₃. These threewavelengths Y₁-Y₃ are arranged on one of the peaks around 550 nm.According to various embodiments, the output signal of the photodetector is a sum of an output signal of the photo detector due toreflected light from the blood B of the user as well as reflected lightfrom the tendons T. The output signal Y_(bt) can therefore beY_(b)+Y_(t), wherein the index “b” corresponds to blood and the index“t” corresponds to tendons. If this equation is applied to three points,the results thereof are as follows:

Y _(1bt) =Y _(1b) +Y _(1t);  (1)

Y _(2bt) =Y _(2b) −Y _(2t);  (2)

Y _(3bt) =Y _(3b) +Y _(3t).  (3)

As may be deducted from FIG. 6, the coordinates of the three points maybe members of an arithmetic series such that

$\begin{matrix}{\frac{Y_{1t} + Y_{3t}}{2} = {Y_{2t}.}} & (4)\end{matrix}$

According to various embodiments, the middle wavelength is at anequidistant position between the first and third wavelength such that

$\begin{matrix}{Y_{2} = {\frac{Y_{1} + Y_{3}}{2}.}} & (5)\end{matrix}$

According to various embodiments, a motion correction can be based onthe following formula:

$\begin{matrix}{Y = {\frac{Y_{1{bt}} + Y_{3{bt}}}{2} - {Y_{2{bt}}.}}} & (6)\end{matrix}$

If the equations 1 to 4 are substituted in the above equation, it can beseen that the influence of the tendons are removed, which results in

$\begin{matrix}{Y = {\frac{Y_{1b} + Y_{3b}}{2} - {Y_{2b}.}}} & (7)\end{matrix}$

As Y_(1b) and Y_(3b) almost correspond to each other, the equation is asfollows:

Y=Y _(1b) −Y _(2b).  (8)

Accordingly, if this formula is used, the influence of the tendons canbe removed such that only the reflected light due to the blood variationis determined.

As an example, the first wavelength Y₁ is 530 nm, the second wavelengthY₂ is 550 nm and the third wavelength Y₃ is 570 nm. Other wavelengthsare also possible as long as the second wavelength is equidistant to thefirst and third wavelength. In other words, the second wavelength isarranged in the middle between the first and third wavelength.

FIG. 8 shows a graph of the output signal of a PPG sensor without andwith a motion correction according to various embodiments.

Other variations of the disclosed embodiment can be understood andeffected by those skilled in the art in practicing the principlesdisclosed herein from a study of the drawings, the disclosure and theappended claims.

In the claims, the word “comprising” does not exclude other elements orsteps and in the indefinite article “a” or “an” does not exclude aplurality.

A single unit or device may fulfil the functions of several itemsrecited in the claims. The mere fact that certain measures are recitedin mutual different dependent claims does not indicate that acombination of these measurements cannot be used to advantage. Acomputer program may be stored/distributed on a suitable medium such asan optical storage medium or a solid state medium, supplied togetherwith or as a part of other hardware, but may also be distributed inother forms such as via the internet or other wired or wirelesstelecommunication systems.

Any reference signs in the claims should not be construed as limitingthe scope.

1. Optical vital signs sensor configured to measure vital signs of auser, comprising: a light source configured to generate light at atleast three wavelengths (Y₁-Y₃) which is directed towards a skin of theuser, wherein the first, second and third wavelengths (Y₁-Y₃) arearranged approximately around 550 nm, wherein the second wavelength (Y₂)is arranged equidistantly between the first and third wavelength (Y₁,Y₃), at least a photo detector unit configured to detect an intensity oflight at the at least three wavelengths, wherein said light isindicative of a reflection of light emitted in or from the skin of theuser, and a motion correction unit configured to correct motionartefacts from the light intensity detected by the photo detector bysubtracting the light intensity detected at the second wavelength froman average of the light intensity detected at the first wavelength (Y₁)and that at the third wavelength (Y₃).
 2. Optical vital sign sensoraccording to claim 1, wherein the second wavelength (Y₂) corresponds toa sum of the first and third wavelength (Y₁, Y₃) divided by
 2. 3.Optical vital sign sensor according to claim 2, wherein the secondwavelength (Y₂) corresponds to approximately 550 nm.
 4. Optical vitalsign sensor according to claim 1, wherein said optical vital signssensor is a photoplethysmographic sensor (PPG).
 5. Method of measuringvital signs of a user with an optical vital signs sensor, comprising thesteps of: generating light at at least three wavelengths (Y₁-Y₃) whichare directed towards a skin of the user, detecting an intensity of lightat the at least three wavelengths (Y₁-Y₃), wherein said light isindicative of a reflection of light emitted from the skin of the user,and correcting motion artefacts from the detected light by subtractingthe light intensity detected at the second wavelength (Y₂) from anaverage of the light intensity detected at the first wavelength (Y₁),and at the third wavelength (Y₃), wherein the first, second and thirdwavelengths (Y₁-Y₃) are arranged approximately around 550 nm, andwherein the second wavelength (Y₂) is arranged equidistantly between thefirst and third wavelength (Y₁, Y₃).
 6. A non-transitory machinereadable medium encoded with instructions for execution by a processor,the non-transitory machine readable medium comprising: instructions forgenerating light at at least three wavelengths (Y₁-Y₃) which aredirected towards a skin of the user, instructions for detecting anintensity of light at the at least three wavelengths (Y₁-Y₃), whereinsaid light is indicative of a reflection of light emitted from the skinof the user, and instructions for correcting motion artefacts from thedetected light by subtracting the light intensity detected at the secondwavelength (Y₂) from an average of the light intensity detected at thefirst wavelength (Y₁), and at the third wavelength (Y₃), wherein thefirst, second and third wavelengths (Y₁-Y₃) are arranged approximatelyaround 550 nm, and wherein the second wavelength (Y₂) is arrangedequidistantly between the first and third wavelength (Y₁, Y₃).